Various forms of variable dampers have been proposed for use in wheel suspension systems for the purposes of improving the ride quality and achieving a favorable motion stability of the vehicle. Typically, such a variable damper may be formed as a tubular shock absorber using magneto-rheological fluid (MRF) for the actuating fluid of the damper so that the viscosity of the fluid may be varied by supplying corresponding electric current to a magnetized liquid valve (MLV) which is incorporated in the piston. The damping force property of such a variable damper can be varied either in a stepwise fashion or, more favorably, in a continuous manner.
For instance, a rolling movement of a vehicle as it makes a turn owing to a lateral acceleration of the vehicle (owing to the inertia force) can be prevented from becoming excessive by increasing the target damping force of the dampers in dependence of the increase rate (time differential) of the lateral acceleration. Also, the rider quality of a vehicle as it travels over an irregular road surface can be improved by reducing the target damping force of the dampers by detecting a rapid vertical up and down movement of the wheels or a high stroke speed of the wheels. Thereby, the vertical movement of the wheels is prevented from being transmitted to the vehicle body. See Japanese patent laid open publication No. 2006-69527.
However, according to the method disclosed in this Japanese patent publication, when the differential value of the lateral acceleration is large (or the target damping force is high) when the vehicle is cornering or changing lanes, the resulting increase in the damper force prevents the movement of the damper that otherwise would insulate the irregularities of the road surface to the vehicle body, and this adversely affects the ride quality of the vehicle.
A vehicle undergoes a rolling movement when cornering a curve or change lanes, and a pitching movement when decelerating. For the vehicle to able to travel in a stable manner, it is desirable to increase the damping forces of the suspension systems when the vehicle is rolling and/or pitching. On the other hand, the damping forces of the dampers are desired to be small for the vehicle to ensure a favorable ride quality on irregular road surfaces. It is conceivable to control the damping forces of the dampers according to the vertical accelerations of the vehicle body such as wheelhouses of the vehicle. However, as it is difficult to determine if any particular increase in the vertical acceleration of any particular vehicle part is due to a rolling or pitching movement of the vehicle or the irregularities of the road surface.
When a wheel moves up and down owing to the irregularities of a road surface, this up and down movement is transmitted to the vehicle body via the suspension system (spring and damper). Therefore, even when a signal to reduce the damping force is forwarded to any particular damper according to the vertical movement of the vehicle body, there is so much response delay that the rider quality of the vehicle may not be improved. Also, when the response speed of the variable damper is not high, this adds to the problem.
During a cornering movement, the vehicle body rolls outwardly of the curve, and the spring of the suspension system of the outer wheel is compressed and produces a greater spring force while the spring for the inner wheel is extends and produces a smaller spring force. As a result, the outer wheel is prevented from readily displacing vertically and this may impair the ride quality while the inner wheel becomes so readily displacing vertically that the control of the rolling movement of the vehicle body may be impaired.